This is a Continuation In Part application of co-pending United States patent application Ser. No. 08/919,907 filed Aug. 28, 1997 in the names of Nicholas, Edward Sargent, et al., and assigned to the assignee of the present invention.
The “lost foam” process is well known in the art, and involves principally the steps of: (1) preparing (e.g., molding) a pattern from a fugitive material such as expanded polystyrene (EPS) foam; (2) coating the pattern with a refractory coating (i.e., about 0.25 mm to about 1.5 mm thick) to stiffen the pattern, provide a barrier between the surface of the pattern and the sand, and control the metal fill rate; (3) drying the coating; (4) investing the pattern in a bed of unbonded sand to form a mold about the pattern; (5) pouring molten aluminum into the mold to vaporize and displace the pattern, and form a casting that replicates the shape of the pattern; and (6) removing the sand from around the casting. The casting may or may not be water quenched in the course of a heat treatment following casting depending on the particular aluminum alloy used and the desired final microstructure. For example, A319 aluminum alloys are frequently quenched in water in the course of a T-5 heat treatment to improve its machineability and dimensional stability.
EPS pattern coatings typically comprise water-based slurries (i.e., about 45-70% by weight solids) including various combinations of (1) refractories such as silica, alumina, chromite, mica, zircon, quartz, olivine, aluminosilicates or hollow glass microspheres, (2) binders such as clay and various polymers, (3) surfactants, (4) thixotropic agents, and (5) dispersants. On a dry basis, the coatings comprise about 80% to about 98% by weight refractory, about 0.5-18% by weight binders, and about 2% by weight surfactants/dispersants/and thixotropic agents. The permeability of the coating is controlled by the size and shape of the refractory particles and serves to control the rate at which liquid and gaseous pyrolysis products escape into the sand which, in turn, controls the rate at which the aluminum enters the mold. The coatings are most commonly applied by dipping the pattern therein, but may also be sprayed, brushed or pumped onto the pattern. Though water is the most common carrier for the refractories, other carriers that are compatible (e.g., a non-solvent) with the pattern may also be used.
Unfortunately, some residue from the refractory coating that is applied to the foam pattern adheres to the surface of the aluminum casting, and is difficult, time consuming and expensive to remove. This is particularly the case where the coating residue needs to be removed from the internal passages of complex castings such as internal combustion engine blocks or heads. Some known techniques for removing at least a portion of the coating residue include (1) thermally shocking the coating by plunging the hot casting into water immediately after casting, (2) shot blasting the casting (e.g., with plastic, steel or ceramic shot), (3) scrubbing the casting in an agitated aqueous slurry of abrasive particles, (4) rapping the casting with a hammer(s), (5) immersing the casting in a bath of ultrasonically agitated water, (6) blasting the casting with a stream of water or steam, and (7) dipping the casting in a bath of molten salt, or caustic soda.